Shoulder surfing, also known as visual hacking, is the activity of obtaining information from or about others by observing visual content of displays that actually should be kept secret, such as PINs, passwords, or private text messages. Approaches that address shoulder surfing on mobile devices mainly focus on ways to recognize observers or to complicate visual presentations for them from the system's perspective. However, users also have developed their own strategies to keep their input secret. With this work, we contribute an investigation of strategies to prevent shoulder surfing from the users' perspective. We performed a user study and observed 32 participants while interacting with smartphones using different kinds of eyes-free device-based interaction techniques. We identified several strategies that users had to prevent shoulder surfing. These strategies help us to develop effective ways to design useful interactions that overcome shoulder surfing issues.